It is known that (cyclo)aliphatic diamines can be phosgenated in the gas phase. In SU-A 00 407 567 the reactor used for this purpose is heated on the outside and on the inside and provided with cooling coils in order to remove the heat of reaction. On the inside of this reactor is a holder for the preheating units around which are arranged pipes through which the reactants are passed. The reactor itself also has external heaters. The preheaters and the external heaters are switched on until the temperature reaches 210° C. to 230° C. A stoichiometric quantity of phosgene is then passed through one pipe, while the gaseous or liquid amine reaches the reactor via the other pipe. The reactor has previously been heated to a temperature of 30° C. to 60° C. above the boiling temperature of the amine. Gaseous isocyanate is then formed in the reactor. The heat of reaction is removed through cooling coils. When the latter have become sufficiently hot, the heater is switched off. The external heater is switched on only if this is necessary in order to compensate for heat losses, so that the product does not cool to below 210° C. to 230° C. The gaseous products leave the reactor through an outlet. The disadvantage of this reactor is that it is suitable only for batch operation.
In GB-A 1 165 831, the reaction is carried out in a tubular reactor equipped with a mechanical stirrer. The reactor resembles a film evaporator in which the stirrer mixes the gases and at the same time brushes over the heated walls of the tubular reactor, in order to prevent a build-up of polymeric material on the wall of the pipe. However, the use of a high-speed stirrer when handling phosgene at a temperature of approximately 300° C. necessitates great expense on safety measures to seal the reactor and retain the stirrer in the highly corrosive medium.
EP-A 0 289 840 and EP-A 0 749 958 describe a cylindrical reaction chamber without moving parts, in which the reactants are reacted with one another while a turbulent flow is maintained. The geometrical form of the cylindrical reaction chamber leads to back-mixing processes, as a result of which the products react with the diamine starting material to form solid deposits. This leads to contamination of the reactor and blockages in the path of the gas.
The phosgenation of aromatic diamines is described in EP-A 0 593 334 in which the reactants are initially mixed in a turbulent flow (at Reynolds numbers of at least 3000, preferably at least 8000). The reaction is then carried out under laminar or turbulent flow conditions in a tubular reactor without movable mixing elements and with a constriction of the walls. The fact that it is possible to phosgenate aromatic diamines cannot automatically be transferred to aliphatic or cycloaliphatic diamines or triamines because the corresponding reactions differ fundamentally from one another with regard to mechanism, solid-forming secondary reactions and required reaction times.
It is an object of the present invention to produce (cyclo)aliphatic isocyanates by the gas-phase phosgenation of the corresponding amines while avoiding the previously mentioned disadvantages of prior art.
This object may be achieved with the process of the present invention by carrying out the reaction in a reactor which differs from the prior art cylindrical form in a specific way, which is described below.